This invention relates to a fluid pressure energy translating device and more particularly to an improved high pressure hydraulic axial piston pump or motor.
Such axial piston pumps or motors generally comprise an annular block or barrel defining a plurality of cylinders arranged concentrically of the barrel axis which each slidably receives one of a like plurality of pistons. The pistons are operatively connected through spherical bearings disposed with shoes to an inclined wobble plate or cam plate disposed adjacent one end of the barrel. The shoes slide on the stationary cam plate as the barrel is rotated. Reciprocation of the pistons in response to relative rotation between the cam plate and the barrel is thus effected. The barrel is supported on a drive shaft for rotation about its axis and about a fixed valve or port plate which engages the end of the barrel opposite the cam plate. The port plate has a pair of ports or passages for connection, respectively, to a source of fluid and to a discharge line. The ports in the port plate register with a plurality of spaced-apart ports in the barrel face which communicate with the individual cylinders so that fluid will be alternately introduced into and discharged from each cylinder as the barrel is rotated and the pistons reciprocate.
In order to maintain a fluid seal between the rotating barrel and the fixed port plate, the mating surfaces must be extremely flat and perfectly parallel. Typically, the mating surfaces are flat to within two lightbands of flatness. Furthermore, proper axial alignment between the barrel and the port plate must be maintained. If the barrel and port plate slightly axially misalign, i.e., tilt relative to one another, increased wear of the mating surfaces on the port plate and the barrel will occur. If the tilt is great enough, system pressure will act on usually unexposed area causing barrel/port plate separation or "blowoff".
This problem is both significant and difficult to solve since not only is it dynamic, i.e., dependent upon speed and other operating conditions but it is also the result of several forces acting upon the barrel. Fluid pressurization and pumping by the pistons is accomplished by interaction between the cam plate and piston shoes. The force which the cam plate exerts on each of the piston shoes to pump fluid is balanced by a reaction force in the opposite direction. Due to the inclination of the cam plate, this axial reaction force produces a radial component of force tending to move the piston shoes radially away from the barrel axis and the cam plate. The forces from each of the piston shoes may be resolved into a single resultant force acting on the barrel and extending radially from the barrel axis at the point of intersection of the barrel axis and the plane of loci of the centers of the spherical bearings. It should be noted that the magnitude of this force is proportional to the hydraulic fluid pressure. It is thus not only dynamic but also independent of the rotational speed of the barrel.
A second force tending to tilt the barrel results from centrifugal force. The centers of gravity of some of the different pistons located around the barrel are axially offset from the diametrically opposed pistons. The centrifugal force on each piston acts through the center of gravity of the piston in a direction radially of the barrel. Since the centers of gravity of some of the pistons are axially offset from others, an unbalanced centrifugal force is applied to the barrel. The centrifugal force on diametrically opposed offset pistons applies a dynamic couple to the barrel which is the product of the centrifugal force acting on one of the pistons times the axial offset of the centers of gravity of the pistons. The magnitude of this couple will vary from zero in the case of opposed pistons in which the centers of gravity are aligned at right angles to the shaft to a maximum value in the case of opposed pistons in their maximum offset position. The magnitude of the couple is also directly related to speed.
A general solution to these problems which has been incorporated into the design of most contemporary axial piston pumps comprehends restraining either the barrel or port plate while permitting the other a certain amount of orientation freedom. Through this approach, barrel-valve plate misalignment which might result in leakage and blow-off is minimized since tilting or skewing of one of the elements may be accommodated by movement of the other. Another approach is disclosed in my prior U.S. Pat. No. 3,126,835. By supporting the barrel on a bearing on a drive shaft extending coaxially through the barrel and by properly locating the bearing, the effects of the dynamic couple formed from centrifugal force may be offset at least in part by the effects of the resultant force from fluid pressure acting on the pistons.
The unequal forces acting on the barrel also tend to slightly deflect the drive shaft which supports the barrel. If the barrel is rigidly connected to the drive shaft and the drive shaft deflects or bends slightly under loading, the barrel will tilt relative to the port plate and a loss of fluid pressure will occur. In my prior U.S. Pat. Nos. 3,126,835 and 3,160,109, a loss of fluid pressure resulting from deflection or bending of the drive shaft is reduced through the use of a torque tube interconnecting the drive shaft with the barrel and through the use of a crowned bearing between the drive shaft and the barrel. The torque tube extends coaxially along the drive shaft between the drive shaft and the barrel and has one end connected through splines to the drive shaft and an opposite end connected through splines to the barrel. As the shaft is driven, the torque tube in turn drives the barrel. The splines between the drive shaft and the torque tube and between the torque tube and the barrel also may be crowned to allow the shaft to flex relative to the barrel without tilting the barrel, as taught in my U.S. Pat. No. 4,232,587. As the drive shaft flexes under loading, the barrel is permitted to slide on the port plate without tilting away from the port plate. This construction has been effective in greatly reducing or eliminating tilting of the barrel and the resulting hydraulic fluid leakage.
In recent years, hydraulic component applications in various industries have become increasingly taxing. For example, axial piston pumps and motors are being asked to far exceed their design capabilities. Increases in both hydraulic pressure and rotational speeds are causing much higher rates of failure in axial piston pumps and motors. Failures primarily occur in the form of barrel-valve plate separation resulting in a loss in pressure of "blow-off" and loss of shoe contact with the surface of the cam plate.
Early axial piston pumps provided a direct drive between the drive shaft and the barrel. In one early design shown in U.S. Pat No. 2,642,810, the drive shaft was connected directly to the barrel through a splined connection having crowned or curved male splines on the drive shaft engaging straight splines on the barrel. As a consequence of the crowned splines, flexing of the drive shaft would not automatically tilt the barrel away from the port plate. However, since no bearing surface was provided between the drive shaft and the barrel, the direction of the tilting forces on the barrel due to centrifugal force and the direction of the tilting forces on the barrel due to fluid pressure acting on the pistons was cumulative, rather than subtractive as in the pumps shown in my above-described U.S. Pat. No. 3,126,835. Consequently, there was still a great tendency for barrel tilting relative to the port plate to occur in the pump shown in the U.S. Pat. No. 2,642,810 under high fluid pressure and high operating speed conditions. Another pump design shown in U.S. Pat. No. 2,915,985 provides both a straight spline connection and a spherical bearing connection between the drive shaft and the cylinder barrel. However, the straight spline connection prevents bending of the drive shaft relative to the barrel so that any flexing of the drive shaft under load is transferred through the splines to the barrel to cause the barrel to tilt away from the port plate.